Modulated backward wave oscillator



Jan. 27, 1959 A, ASHKIN EAL 2,871,451

MODULATED BACKWARD WAVE OSCILLATOR l BV/QQT I A7' TURA/EV MODULATED BACKWARD WAVE OSCILLATOR' Arthur Ashlsin, Irvington, and Laurence R. Walker,.

Bernardsville, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 21, 1953, Serial No. 399,284

4 Claims. (Cl. 332-25) tent of operating wavelengths long past a wave interaction circuit of the kind which is characterized in that an electromagnetic wave propagating therealong in one direction has strong components thereof which have phase velocities in the direction opposite to that of wave energy propagation. It is characteriitic that the noise components in the electron beam induce wave disturbances which propagate along the interaction circuit in a direction opposite to that of electron ow and which, accordingly,

have components whose phase velocity is in the direction of electron flow. For oscillations at a given frequency the velocity of the electron beam is adjusted to provide interaction with a component traveling in the direction of flow ofthe backward-traveling noise-induced wave of the desired frequency whereby the wave is amplified and the electron beam is hunched correspondingly. Because the directions of wave energy propagation and electron ow are opposite, the electron beam provides positive feedback coupling, acting to return energy from points of higher level to points of lower level along the path of tiow. When the beam current is sufficiently high, this action will sustain oscillations and oscillatory wave energy of the desired frequency becomes available for abstraction at the up;tream end of the interaction circuit. Throughout the application, the terms upstream and downstream will be used to designate relative separa` tion from the electron source, the term upstream denoting proximity along the path of liow to the electron source, and the term downstream denoting remoteness along-the path of 'flow from the electron source.

It is characteristic of oscillators of this kind that the electrons are injected unbunched at the upstream end of the interaction circuit where the signal level is high and emerge strongly bunched at the downstream end of the interaction circuit where the signal level is low. rllhis means that the interaction along the path of iiow is a1- ways relatively low, which in turn results in operation which is relatively inefficient. Any attempt to increase the interaction between the beam and the signal wave which involves feeding back directly wave energy from the upstream to the downstream end of the circuit or bunching the electrons at the upstream end of the circuit suffers from the difficulty of phasing; that is, at certain frequencies this feedback will enhance interaction but at others reduce it. Such moding would deprive the backward wave oscillator of one of its most useful properties, i. e., the ability to be electronically tuned over a continuous wide range of frequencies.

The primary object of vthe present invention is to improve the. etliciency ofA backward -wave oscillators. Related objects are to make available in such oscillators high genau Etaient-eci dan. 2:7, lfl

level output oscillatory waves with shorter interaction circuits and/or lower beam currents.

Another object is to make available an improved form of frequency modulator.

To these ends, afeature of the invention is the use in a backward wave oscillator of two distinct oppositely directed electron beams, the first beam to induce in the interacticn circuit a growing electromagnetic wave of a desired frequency in the manner characteristic of backward wave oscillators known hitherto, and the second beam to interact with and amplify further this growing electromagnetic wave in the manner characteristic of forward wave amplifiers known hitherto. In conjunction with the two beams, there is utilized a wave interaction circuit of the kind which is characterized in that a wave propagating therealong'gives rise to strong spatial harmonic components thereofwhich have phase velocities both in the direction of and opposite to that of wave propagation and which is suited for the projection therepast of two distinct electron vbeams for coupling to the propagating Wave.

ln operation, the primary beam interacts with a spatial harmonic component of a noise induced wave of a desired frequency which has a phase velocity in a direction opposite to that of wave propagation for setting up the oscillatory wave which grows with travel in the direction opposite to that of electronflow. The secondary beam is adjusted. to interact with the spatial harmonic component of'the'oscillatory wave which has a phase velocity in the direction of wave propagation thereof. Thus, this secondary beam will serve to amplify the growing oscillatory wave, resulting in stronger interaction between this wave and the primary beam and thereby enhanced amplification .of this. oscillatory wave. This enhancement, in turn, will improve the interaction between the oscillatory wave and the secondary beam. It can be seen that the Veffect is cumulative, resulting in an improvement considerably greater than would have been obtained by utilizing as the current in the primary electron beam the sum of the currents in the two beams. Moreover, the increased eiiiciency of interaction makes possible a sh0rtening of the length of the interaction circuit. f

In an oscillator of this kind, it is important to insure that the secondary electron beam does not set up a second oppositely traveling oscillatory wave of its own. For this purpose, it is advantageous -to make the interaction circuit nonreciprocal in its propagating characteristics, or alternatively, to operate the two beams at velocities suiiciently different that for. the interaction circuit employed the velocity of the secondary beam will not be suited for inducing an oppositely traveling oscillatory wave ofits own.

In one illustrative embodiment the interaction circuit comprises a rectangular wave guide which is folded back and forth on itself a plurality of times in serpentine fashion. By the insertion of suitably magnetized gyromagynetie septa in successive folds of the wave guide, the

circuit is made to offer high attenuation to Waves propagating therethrough in a first direction and low attenuation to waves propagating therethrough in the opposite direction. A primary electron beam'is projected through successive folds of the wave guide in the direction corresponding to that of high wave attenuation for inducing in the circuit a backward traveling oscillatory wave. A secondary electrony beam is projected through successive folds of the wave guide in a direction opposite to that ofthe primary beam for amplifying this oscillatory wave.

In applications as a frequency modulator, the modulating intelligence is used to vary the velocity of the primary electron beam past the interaction circuit, which action has the effect of varying the frequency of the oscillations. Where the oscillatory frequency is to be v s modulated over a wide frequency range,.it may .be advantageous to vary simultaneously the velocity of the secondary beam to insure maximum amplification of the growing oscillatory wave.

In another illustrative embodiment, the wave circuit comprises a wave guide which is l'oadedby'a linear array of wires and the two electron beams ,are .projected past on` opposite sidesof the linear array.' 4With sucha wave circuit, the phase velocities ofnthhe forward and backward traveling componentspf a'wave of the'de's'iregdl frequency can conveniently be made so differentv that the velocity spread'of the two beamswill be sufficiently large that only a chosen one will set up a backward traveling growingV oscillatory wave.

The invention will be better understood from the following more detailed description taken in conjunction with' the accompanying drawings in which:

Fig. 1A shows in perspective cutaway view an illustrative embodiment of the invention which utilizes a folded wave guide circuit throughwhich'areprojected two oppositely directed electron beams, and Fig. 1B is` a transverse section taken along the lines 1B-1B of the embodiment shown in Fig. 1A;

Fig. 2A shows in perspective cutaway view another embodiment of the invention which utilizes as the wave circuit a wave guide loaded with a linear array of wires onopposite sides of which ow two oppositelyl directed electron beamsjand Fig. `ZB is a transverse section taken along the 'linesZB-ZB of the embodiment shown in FigQZA.

'vWith' particular reference now to the drawings, in the traveling wave tube oscillator 1t) shown in Figs. lA and lB,`a rectangular hollow'wave guide 11 is folded back and forth' on itself a plurality of times to form a serpentine wavefpath. "The broad dimension of Vthe, Wave guidev is` normal to the direction of 'folding Whiler the narrow'dim'ensionjof the wave guide is.' parallel to the direction offolding. `:'Ea'ch, fold of the Wave guide is prov'idedj'vvith twoV distinct apertures 12A and 12B, the, various apertures forming two sets of aligned apertures on oppositel sides 'ofthe tube axis for passage therethrough ofseparateelectron beams. To this end, electron sources 13` and 14'at opposite ends of the evacuated glass, envelope 15 which houses the tube elements are each aligned with a different set of apertures and provide electron beams for `low through successive folds. In `target relationshipgwith electron sources 13 and 14 at opposite ends of the Yenvelope are the collector electrodes 15 and 16, respectively. Flux producing means external to the envelope 'are employed to create a longitudinal magnetic fieldA parallel to the paths of flow for focusing the two electron beams. In the embodiment illustrated, a solenoid 17. surrounds the tube envelope to provide the .focusing magnetic ux. 11'is of a non-magnetic material, such as` copper, so as not to disturb this magnetic eld. The output oscillatory wave energy is abstracted by wayof an output coupling connection 18 Vto the wave guiding circuit at its `end adjacent the electron source 13 which serves as the source of the primary beam. At the opposite end, the wave guiding circuit is terminated in a substantially reectionless manner by the insertion of the tapered dielectric block Z which is coated with aA layerof lossy material such a aquadag. Various possiblevforrns of coupling connections and, terminations will be known to 'workers in the art.

In operation, the noise on the primary electron beam serves to induce in the wave circuit electromagnetic waves which proceed therethrough in the direction from the terminated end toward the outputend and so pposite to the direction of flow ofthe primary beam. The velocity of this electron beam is chosen to favor frequencywhereby this favored wavegrws Awithftravel towards the output coupling connection v18. For such interaction, the velocity of the electron beam is made vsubstantially equal to the phase velocity inthe direction of flow of a component of the wave of desired frequency which is advancing through the wave guide 11 in the direction opposite to that of electron flow. Such interaction may be described as interaction with a forward traveling spatial harmonic of kthe backward traveling wave.VY `Henceforth, for the sake of convenience the ;direction Vof flow of the primary electron beam will be termed the forward direction, and the opposite direction the backward direction. Alternatively, this may `be viewed as adjusting the velocity of ow. of thefprimary beam so that each group of electrons thereof sees substantially the same phase of the electric field of the 1oppositely propagating wave in successive wave guide folds. Because the direction of electron flow is opposite to the direction of wave propagation, the primary electron beam serves to provide positive feedback, returning energy from regions of high level to regions of low level. When the current in this electron beam is sufficient, oscillations are zset up at the desired frequency. lThese are the ,opl erating principles of backward wave oscillators ofthe kind `known hitherto.

The presence of the secondary electronk beam which is providedby electron source 14 serves to reduce 'the value of the starting current needed to initiate and sustain these backward wave type oscillations and to increase the output level .of the oscillations. The velocity of the secondary electron beam is adjusted to interact cumulatively with the noiseV induced oscillatory wave. For interaction, the velocity of this electron beam is made substantially equal to the phase velocity in the direction of ow 'of aA Such interaction* isv rf the kind normally used in the usual spatial harmonicamplier and for present purposes will be described asinteraction with a backward component of'vthe backward component of the oscillatory wave.

traveling oscillatory wave. "Because'the primary and secondary beams are. interacting with-forward and backward travelingcomponents, respectively, of the same backward -traveling,wave, their two velocities willnot only be in.

i opposite directions but alsogenerally of differentmagnitu des. v

Alternatively the operation may be described as'folj' lows. The primary electron beam is providedwith a velocity which results in interaction with the negativerv space harnionic of the oscillatory wave while the secondary beam is provided with a velocity which results in interaction with a positive spaceharmonic of theoscillatory wave, where the positive and'negative spaceharmoniesA have phase velocities, respectively, in the..direc tion of, and the direction oppositeof, that oflthefgroup.

velocity of the oscillatory wave.

They secondaryelectron beam often. will vhavesa tendency to induce growing noise wavesof-itsown fontravel-A ing upstream therealongaif :the wavefcircuitais broad band. Forl this reason,`it isadvantageous,tol'makettheL wave'circuit 11 nonreciprocalin its attenuation charac*l teristics whereby the attenuation toV a wave propagating in the direction from the terminated end of theswave. circuit 11 to the output coupling connection 18is loW,`

while the attenuation to waves propagating in the direction from the output coupling connection to the terminatedend is much higher. In a copendingapplication. Serial No..399,252, filed December 21, 1953, vby R. Kompfner,

row sidewall of the wave guide andthe wave guide axis extending longitudinally parallel to the WaveV guideaxis.l In the ,presence of the longitudinal magnetic 4field used for j. focusing; .thefelectron beam;septanrsospositionedtwilll be magnetized in a transverse direction parallel4 to the electric vector in the wave guide. Since the direction of low attenuation is related both to the direction of the biasing magnetic field and on which side of the wave guide axis the various septa are positioned, it becomes necessary to dispose the septa in adjacent folds on opposite sides of the wave guide axis since the direction of wave propagation relative to the direction of the longitudinal magnetic field reverses with successive folds. To illustrate the shift in position of the sept'a relative to the tube axis with successive folds, one portion of the tube is shown cut open along the tube axis while another portion is shown cut open along a side wall.

It is characteristic of an oscillator of this kind that the frequency of oscillations will be controlled by the velocity of the primary electron beam through the' wave circuit, which velocity is controlled by the potential difference between the electron source 13 and the wave circuit 11. This voltage is controlled by the voltage supply source 27 connected therebetween. The voltage source 2S connected between the electron source 14 and the wave lcircuit controls the velocity of the secondary beam past the wave circuit and this is adjusted for maximum gain of the oscillatory wave.

For frequency modulation operation, a source 29 of modulating voltage under the control of modulating intelligence is connected serially with the voltage source 27 for varying the accelerating potential acting on the primary beam. In instances where the wave circuit 11 is highly dispersive, variations in the frequency of the oscillatory wave result in variations of the phase velocity ofv the components interacting with the secondary electron beam. To maintain the condition for maximum interaction between the secondary beam and the growing oscillatory wave over a wide range of frequencies, it becomes necessary to vary the velocity of this secondary electron beam as the frequency of oscillation is changed. In general, the magnitude of the two beam velocities should advantageously be varied in the same direction simultaneously. Accordingly, a source 30 of modulating voltage similarly under the control of the modulating intelligence is connected serially with the voltage source 28 for varying the accelerating potential on the scondary beam in synchronism with changes in the accelerating potential on the primary beam.

Figs. 2A and 2B show a backward wave oscillator 40 utilizing a wave circuit which is of narrower band than that used in the tube shown in Figs. 1A and 1B so that it becomes less important to make the Wave circuit substantially unidirectional in its transmission characteristics.

The various tube elements are house in an evacuated envelope 41. The wave circuit comprises a hollow wave guide 42 of rectangular cross section within which, centrally located, extend two rectangular sections 43 and 44, each having one wall 43A, 44A ush with a broad wall of the wave guide 42 and the wall 43B, 44B opposite thereto formed of a linear array of spaced wires. Of course, portions of the walls of wave guide 42 may serve as walls 43A, 44A. The two sections 43 and 44 each extend approximately half way across the wave guide 42 and the wires forming their walls 43B and 44B are interleaved to form one substantially linear array of wires, successive wires of the array being integral alternately with sections 43 and 44. As a result, waves traveling through the wave guide 42 induce currents in adjacent wires of the linear array formed by walls 43B, 44B which are in opposite directions so long as the uniform spacing between the wires in the linear array is appreciably less than one half the wavelength of the wave in the wave guide 42. Accordingly, there will be an electric field set up between adjacent wires whose direction reverses with succeeding gaps between adjacent wires and which, accordingly, is well suited for interaction with an electron beam. The characteristics of such an array of wires as 6 t a wave circuit are described more fully in L. RcWalker Patent 2,746,036 issued May 15, 1956. 1

Electron sources 45 and 46 at'opposite ends of the en- Velope provide the two distinct electron beams and collectors 47 and 48 serve as their respective targets. The electron source 45 serves to provide the primary beam which is employed to set up the growing oscillatory wave while the electron source 46 provides the secondary beam used for amplificationof the growing oscillatory wave. The spread in phase velocities of the backward and forward traveling components of the oscillatory wave will generally be wide and so the spread in V'velocities yof the two beams needs similarly to be wide. As a result,

Vthe velocity of the secondary beam will not be in the range suited for establishing a growing wave of its own,

wave guide 42. To illustrate the manner of the merger a portionfof the envelope at the output end of the tube is cut away. As can be seen, rst thek wire walls 43B,

44B gradually disappear, then the walls connected there.

to are tapered till they vanish also.

For operation as a frequency modulator, provision canl be `made for modulating the beam accelerating voltages in the manner described in connection with the oscil-` lator shown in Fig. lA.

- it should be evident from the embodiments illustrated thatthe principles of the inventOIl can be applied to backward wave oscillators using diverse forms of wave.

circuits.

What is claimed is:

l. In an oscillator, a wave interaction circuit a plurality of operating wavelengths long of the kind which is characterized in that a wave propagating therealong in a given direction gives rise to components thereof with phase velocities both in the direction of and direction opposite to that of wave propagation, a first electron source at a first potential for forming a primary electron beam for tiow past said interaction circuit for interaction with a predetermined component of a wave of desired frequency having a phase velocity in the direction opposite to that of wave propagation, a second electron source at a second different potential for forming a sec ondary electron beam spaced apart from said primary electron beam for flow past said interaction circuit in a direction opposite to that of the primary electron beam for interacting with a component of the wave of desired frequency and having phase velocity in the direction of the wave propagation, dissipative termination means at the downstream end of said interaction circuit relative to said first electron beam, and means coupled to the end of the interaction circuit upstream along the primary electron beam for abstracting for utilization oscillatory wave energy.

2. In an oscillator, a wave interaction circuit of the kind which is characterized in that a wave propagating therealong in a given direction gives rise to components thereof with phase velocities both in the direction of and direction opposite to that of wave propagation, means inserted along said interaction circuit for offering low attenuation to waves traveling therealong in the given direction and high attenuation to waves traveling therealong in the direction opposite to the given direction, means for forming a first electron beam for flow past said interaction circuit in the direction opposite to the given direction for interacting with a component of a wave or" desired frequency having a phase velocity in the direction opposite to the given direction, means for forming a second electron beam spaced apart from said first electron beam for fiow" past said interaction circuit in the given direction for.' interacting with a component of the wave of desired frequency and having a phase l velocity in thev given direction,y dissipative termination means a't the downstream end of saidinteractionv circuit relative' to said. first electron beam, and means for abstiracting Awave energy coupled to the end of the interaction circuit upstream along the first beam.

3. In a frequency modulator a wave interaction circuit of the kind which ischaracterized inr that va wave propagating'therealong in a given direction gives rise to spatial harmonic components` thereof with phase velocities both in 'the direction' of and direction opposite to that `of wave p propagation, means for'formin'ga first electron beam o'f. a first velocity for flow past said interaction circuit in' the given direction for interactingV with a predetermined spatial harmonic component of a wave of desired frequency and having a phase velocity inthe direction opposite to that ofwave-lpropagation, means for forming al second electron beam spaced apart from said first electron beam and` of a different second velocity for fiow pastv said interaction circuit in a directionopposite to that of the first electron beam for interacting with a predetermined 'spatial harmonic component of the wave a'given pass band of frequencies of theA kind which is characterized in that a wave propagating there'along in a given direction gives rise to negative and positive space harmonics,.means'for forming a first electron beam for flow pa'stsaid interacton circuit overa distance a'plurality of. operating wavelengths longv for interacting. with a-negative space harmonic-*of .awave ofwdesired frequency Iin the-pass band,'means forf'orminga second4 electron .beam spaced apart from said rfirst electron beam for flow past said interaction circuit over a distance a plurality of 0pe'ratingtfwavelengths long in ardirection opposite to that of. the first electron beam forv interacting with a positive space/harmonic ofthe wave of desired frequency, dissipative termination means at the downstream end of said v interaction circuit relative to said first electron beam,

and means coupledto the end of the interaction circuit upstream along the -firstelectron beam for abstracting for utilization wave energy.

References Cited 'inthe file o f this patent UNITED STATES PATENTS '2,586,816 Hansen 'et ai. v Feb. 25,1952 '2,603,773 Field 2 t July 15, v1952 2,630,544 Tiley v r v Mar. 3, 1,953 2,654,047 Clavier Sept. 29, 1953 2,730,647 Pierce Jan. l0, 1956 27,757,311 Huber et al. Juiy 31,1956

FOREIGN PATENTS y 993,102 France Q July 18, 1951 OTHER "REFERENCES Article .by E. C. Dench, Tele-Tech, November 1953, pages 64-66 and IS7-162.

Proceedings of the IRE, ,.pp. 478-482, April 195,2. 

